316 THE STRUCTURAL EVIDENCE OF EVOLUTION". 



Vertebrata. In a number of groups tbe embryo seems to have 

 been more susceptible to the influence of the environment than 

 the adults.* It results that in many cases the phylogeny can 

 only be determined by the discovery and investigation of the 

 ancestors themselves, as they are preserved in the crust of the 

 earth. In all cases this discovery confirms and establishes such 

 definite conclusions as may be derived from embryology. It is 

 also clear that on the discovery of phylogenetic series it becomes 

 at once possible to determine the nature of defective types. It 

 becomes possible to ascertain whether their rudimental parts 

 represent the beginnings of organs, or whether they are the 

 result of a process of degeneration of organs once well developed. 



A great deal of light has been happily thrown on this question, 

 as regards the Vertebrata, by the recent work done in North 

 American paleontology. The lines of descent of many of the 

 minor groups liave been positively determined, and the phyloge- 

 netic connections of most of the primary divisions or classes have 

 been made out. The result of these investigations has been to 



* A remarkable instance of this state of things appears in the history of the 

 evolution of the insects. It is quite impossible to understand this history without 

 believing that the larval and pupal states of the highest insects are the results of a 

 process of degeneracy which has affected the middle periods of growth but not the 

 mature results. The earliest insects are the Orthoptera, which have active aggres- 

 sive larvae and pupae, undergoing the least changes in their metamorphosis (Ameta- 

 bola), and never getting beyond the primitive mandibulate condition at the end. 

 The metamorphosis of the jawed Neuroptera is little more marked, and they are 

 one of the oldest orders. 



The highest orders with jaws undergo a marked metamorphosis (Coleoptera, 

 Hymenoptera), the Hymenoptcra even requiring artificial intervention in some in- 

 stances to make it successful. Finally, the most specialized orders, the suctorial 

 Diptera and Lepidoptera, especially the latter, present us with very unprotected 

 more or less parasitic stages, both active and inactive. These animals have 

 evidently degenerated, but not so as to prevent their completing a metamorphosis 

 necessary for purposes of reproduction. As is well known, many imagines (Satur- 

 niidae, Qlstridce) can perform no other function, and soon die, while in some Diptera 

 the incomplete larvae themselves reproduce, so that the metamorphosis is never 

 completed. 



This history is parallel to that proposed by Dohrn to account for the origin of 

 the Ammocoetes larval stage of the Marsipobranchii. He supposes this form to be 

 more degenerate than its probable ancestral type in the ancestral line of the Verte- 

 brata, as it is inferior to its own adult. An inactive life in mud is supposed by 

 Dohrn to have been the effective cause. An inactive life on the leaves of plants, 

 or in dead carcases, has probably been the cause of the same phenomenon in the 

 Lepidoptera and Diptera. 



